In our above-identified copending application we have disclosed and claimed a system of this general type using a bus common to a multiplicity of terminals or stations operating in a broadcast mode, each station being able to pick up all the messages transmitted over the bus but actually extracting only those that are specifically addressed to it. An interface at each station comprises a first or lower-tier portion (DLC) with wired-logic circuitry and a second or higher-tier portion (VPP) with microprogrammed circuitry. The first portion (DLC) temporarily stores outgoing information to be sent over the bus as well as incoming information received from the bus and addressed to the station involved, this information being in the form of digital message frames. The second portion (VPP), under the control of a microprocessor, organizes the establishment of virtual point-to-point links between the station referred to and other stations.
As further described in our copending application, whose disclosure we wish to incorporate by reference into the present one, the second interface portion (VPP) may concurrently handle several virtual links extending between its own station and respective other stations reached through the bus. These links are being handled independently of one another as concerns their initialization, termination and possible re-initialization. One or more packets with different ultimate destinations may be encompassed in a single outgoing message frame. Several frames, especially short ones, may be successively sent out in a single binary sequence after access to the bus has been obtained. Stations competing for such access may avail themselves, according to the teachings of our copending application, of a contention protocol of the type known as CSMA-CD (for Carrier-Sense Multiple-Access Collision Detection) with presistent probability. Such a protocol has been described, for speech-signal transmission, in a paper titled "A Local Access Network for Packetized Digital Voice Communication" by Daniel H. Johnson and Gerald C. O'Leary, presented at the 1979 IEEE National Telecommunication Conference held Nov. 27-28, 1979 in Washington, D.C. Reference may also be made to an article titled "A Carrier Sense Multiple Access Protocol for Local Networks" by Simon S. Lam, published on pages 21-32 in Vol. 4 of COMPUTER NETWORKS (1980), and to a further article titled "Performance Analysis of Carrier Sense Multiple Access with Collision Detection" on pages 245-259 of the same publication.
In another commonly owned copending application, Ser. No. 425,858 filed Sept. 28, 1982 by one of us (Maurizio Sposini) jointly with three others, there has been disclosed a generally similar communication system in which the CSMA-CD contention protocol is used for both speech and data transmission by stations competing for access to the bus. A station having obtained access is allotted a time slot of suitable length in a recurrent frame to be utilized for sending out speech packets. The part of each frame not used for voice communication is available for data transmission. The duration of a frame corresponds to the recurrence time of consecutive packets relating to one and the same voice communication; thus, the allocation of homologous periods of successive frames establishes a synchronous channel for such communication. Such synchronism, on the other hand, is not required for data transmission. In a specific instance given in that pending application, a frame has a duration of 20 ms sufficient to accommodate a considerable number of 1000-bit speech packets at a transmission rate of 30 Mbits/sec, with time left over for data transmission in a residual period.
As further described in application Ser. No. 425,858, whose disclosure we also wish to incorporate by reference into the present one, the emission of an actual speech packet is preceded by a "booking" or pre-engagement packet of like duration designed to reserve a time slot in an available frame position for the establishment of a speech-communication channel. A state-of-line memory unit stores a "map" reflecting the activity of the bus in each frame, i.e. the distribution of pre-engagement, speech and data packets therein, thus providing information on frame time available for new bookings.